Water-thinnable polyesters can be obtained by reacting hydroxy-functional polyesters with acid anhydrides, thus incorporating carboxyl end groups into the polyester, which are converted after neutralization into salt groups having a hydrophilically modifying action. Products of this type are described, e.g., in EP-A 0 330 139 and DE-A 37 39 332, the entire contents of each of which is hereby incorporated by reference. Since the carboxyl groups incorporated in this way can exhibit an increasing tendency to saponify with rising pH value in aqueous solution or dispersion, these products are dissolved in relatively large quantities of organic solvents and are not dispersed in water until shortly before the production or application of the paint. In this way, possible problems based on their susceptibility to be saponified are minimized. However, this minimization is gained at the cost of a very high content of organic solvents, which no longer meets current requirements for low-emission coatings. Moreover, the subsequent dispersing step is relatively laborious and complex.
Aqueous polyester dispersions are often produced by reacting hydroxy-functional polyesters with hydroxycarboxylic acids and polyisocyanates. In this way, the incorporation into the polymer of the acid groups necessary for hydrophilic modification can be achieved via hydrolytically stable urethane groups. Products of this type are described, e.g., in EP-A 0 498 156, EP-A 0 496 205, DE-A 39 36 288 and DE-A 33 45 618, the entire contents of each of which is hereby incorporated by reference. These urethane group-containing polyester dispersions usually exhibit good storage stability, as well as good mechanical and/or optical film properties. It is a disadvantage, however, that to produce these urethane group-containing polyester dispersions, relatively large quantities of solvent are generally needed, which then have to be distilled off again. In addition, the products have significantly higher molecular weights than pure polyesters, which can lead to disadvantages in pigment wetting, in the optical film properties and also in adhesion to critical substrates compared with pure polyesters.
Aqueous polyester-polyurethane dispersions which are simple to produce, require no unusual polyester raw materials, exhibit very good pigmentability and which permit the production of high-quality clear lacquers, paints and coatings with very good flow, variably adjustable hardness/elasticity level and resistance properties and good stone chip protection and anti-corrosive properties, would be advantageous. The dispersions should also be stable when stored at 40° C. in combination with various hardener resins, with no major changes in viscosity or sedimentation occurring. Such viscosity change and/or sedimentation can occur when the dispersion and the hardener are not sufficiently compatible, and/or when slow crosslinking reactions take place.